1. Field of the Invention
The present invention relates to a switching power supply, which is a flyback switching power supply detecting a voltage generated in a tertiary winding of a transformer to carry out control on the basis of the detected voltage so that the value of an output voltage of a secondary winding of the transformer becomes a specified value, and the control circuit of the switching power supply.
2. Background Art
Previously, for a power supply system for a battery charger and a power supply system for an AC adapter for a notebook computer, a switching power supply system has been used which carries out control so that a supply voltage for a load becomes constant. Moreover, with the trend of promoting energy-saving, reduction in stand-by power, consumed with an electrical appliance being connected to an outlet, becomes a major problem.
One measure for solving the problem is to reduce a switching frequency when an electric appliance is in a light load state or a no load state as a stand-by state, by which a switching loss is reduced to reduce the stand-by power consumption. In JP-A-2003-079146, for example, a measure for reducing stand-by power consumption is described. According to the measure, an intermittent operation is carried out in which a series of switching pulse signals are continuously supplied when the value of a feedback voltage on the secondary winding side output becomes smaller than a specified value and the supply of a series of the switching pulse signals is stopped when the value of the feedback voltage becomes larger than the specified value. The measure is effective in an output voltage control system in which a secondary winding side output voltage is directly fed back by using an optical insulating device such as a photocoupler.
In JP-A-2006-136034, for example, a measure like the following is described. According to the measure, when it is decided by an output signal of a photocoupler that a load is a light load (an output voltage is high), a switching operation by a switching device is stopped for a certain specified period and, when it is decided in the period of making the switching operation stopped by an output signal of the photocoupler that the output signal is lowered, the switching device is turned-on by a trans-resetting detection signal outputted when a resonant voltage on the primary winding side becomes zero as a result of detection of the output of the tertiary winding after the switching operation is brought into the period of being stopped.
However, a photocoupler is expensive and has more than a few problems with respect to long-term reliability. Therefore, a switching power supply system is desirably formed without using a photocoupler. For a measure implemented in such a system, a secondary winding side output is detected from a voltage such as a voltage of a tertiary winding to control an output voltage. The measure, however, has the following problems.
First, in the measure of reducing stand-by power consumption by decreasing a switching frequency into a stand-by state, as is shown in FIG. 6 (a timing chart showing operations of a related switching power supply system), for preventing a faulty operation due to a surge current at the turning-on of a switching device, the minimum width of the turning-on time in the switching operation is widely determined to some extent. This is to cause the amount of energy determined by the minimum width of the turning-on time and the lowest frequency in the switching operation to be transmitted to the secondary winding side. The amount of the transmitted energy, however, cannot be consumed when the secondary winding side is provided with a light load or no load to cause an increase in the output voltage, by which a problem occurs in that no output voltage can be controlled to be at a constant level.
To prevent this, a bleeder can be connected across the output side for suppressing an increase in the output voltage to keep the level of the output voltage at a constant level. In this case, however, a power loss in the bleeder becomes large enough to make it impossible to reduce stand-by power consumption.
Moreover, in the system of carrying out an intermittent operation described in JP-A-2003-079146, there is the presence of operation stopping periods because the switching operation supplying a series of the switching pulse signals is stopped when the value of the feedback voltage on the secondary winding side becomes larger than a specified value as was explained in the foregoing. In the system, a primary winding control is carried out in which information on an output voltage on the secondary winding side is detected by carrying out a switching operation on the primary winding side. Thus, once the switching operation is stopped, no output voltage can be detected for some time. This causes a problem in that the switching operation is kept stopped until the voltage across a holding capacitor in a sample hold circuit, included on the secondary winding side, decreases to be below an instruction value to considerably lower the output voltage.
A system of making an intermittent operation (a burst operation) carried out by using no photocoupler in a switching power supply using a transformer will be capable of determining a period during which an operation is stopped with the use of a timer as is described in JP-A-2006-136034. In the system, however, the period of stopping an operation is individually determined depending on the condition of a power supply system being used, posing the problem that a short period of stopping the operation makes energy-saving impossible and a long period of stopping the operation makes it impossible to ensure a required output voltage.